Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean

Abstract

We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South Amenca in autumn 1996. Each diatom cell contained the heterocystous. N2-fWng cyanobacterial endosymbiont Richeiia intracellularis. Surface Richeiia heterocyst (and filament) densities increased from <I00 to >106 heterocyst 1-' in the bloom. Total abundance ranged from 10"eterocyst m-2 outside the bloom to over 10" heterocyst m-2 within the bloom. Rates of primary production averaged 1.2 g C m-2 d-', higher than typical for oligotrophic Open ocean waters.Nz fixation during the bloom by the Richelia/Hemiaulus association added an average of 45 mg N m-' d-' to the water column. The relative importance of NH,+uptake over the Course of the bloom increased from 0 to 4 2 % of total N uptake by the Hemiauluslficheiia association. N2 fixation by Richelia exceeded estimates of 'new' N flux via NO3 diffusion from deep water and, together with additional N, fixation by the cyanobacterium Trichodesmium, could supply about 25% of the total N demand through the water column during the bloom. Suspended particles and zooplankton coliected within the bloom were depleted in "N, reflecting the dominant contribution of N2 fixation to the planktonic N budget. The bloom was spatially extensive, as revealed by satellite imagery, and is calculated to have contnbuted about 0.5 Tg N to the euphotic zone. Such blooms may represent an irnportant and previously unrecognized source of new N to support pnmary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates that heterocystous cyanobacteria can also make quantitatively important contributions of N in oceanic water column environments.

Morphology and DNA content of bacterioplankton in the northern Gulf of Mexico: analysis by epifluorescence microscopy and flow cytometry

“…data). Surface blooms of Trichodesmium are widespread and abundant in the open Gulf (Biddanda & Benner 1997) and account for ≤ 25% of total new nitrogen input in the north Pacific gyre (Karl et al 1995, Letelier & Karl 1996 and the tropical Atlantic Ocean (Carpenter et al 1999). Nitrogen input from Trichodesmium may enhance the rate of heterotrophic utilization of DOM (Biddanda & Benner 1997).…”

In situ transcriptomic analysis of the globally important keystone N2-fixing taxon Crocosphaera watsonii

“…C. watsonii is widely distributed in tropical surface waters of the northwestern Atlantic and Pacific oceans, where it is typically present at abundances o10 3 cells per ml (Church et al, 2005;Zehr et al, 2007b). Although blooms of large colonial (for example, Trichodesmium) (Capone et al, 1997;Carpenter, 1983) and symbiotic (Carpenter et al, 1999) diazotrophs have been reported in tropical waters worldwide (Capone et al, 1997), unicellular cyanobacteria are less easily observed (Zehr et al, 2008), so little is known about the variability in abundance of these microorganisms.…”

Seasonal dynamics of the endosymbiotic, nitrogen-fixing cyanobacterium Richelia intracellularis in the eastern Mediterranean Sea

“…Diatomdiazotroph associations containing the cyanobiont R. intracellularis and the filamentous non-heterocystous Trichodesmium spp., were traditionally considered the dominant nitrogen-fixing plankton in marine tropical oceans. The 'New N' supplied by these organisms, and a variety of small unicellular diazotrophs described recently (Zehr et al, 2001;Montoya et al, 2004;Zehr et al, 2007), contribute substantially to these systems (Venrick, 1974;Mague et al, 1977;Carpenter and Romans, 1991;Capone et al, 1997;Carpenter et al, 1999;Arrigo, 2005). In the tropical North Atlantic, extensive N 2 fixation by blooms of diatoms and R. intracellularis produced nearly 70% of total N demand in surface waters Janson et al, 1999).…”